1. Field of the Invention
The present invention relates to new catalysts for the oxidation of unsaturated aldehydes to produce carboxylic acids and to methods of making and using such catalysts. More particularly, this invention relates to mixed metal oxide catalysts for the oxidation of unsaturated aldehydes, such as acrolein, to produce the corresponding alpha-beta unsaturated carboxylic acids, i.e., acrylic acid.
2. Description of the Prior Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
The vapor phase oxidation of acrolein for the production of acrylic acid is known in the art. Various references disclose the use of mixed metal oxide catalysts containing molybdenum and vanadium, alone or in combination with other metals, for use in the catalytic oxidation of acrolein. The following is a brief summary of the disclosures of several of these references.
Japanese Patent Publication No. 11343261 discloses the preparation of acrylic acid by catalytic oxidation of acrolein in the presence of a metal oxide catalyst. The catalyst has the formula MoViSbjAk, wherein A is Nb or Ta, i and j are 0.01-1.5, the ratio of j to i is 0.3-1.0, and k is 0.001-3.0. Acrolein is oxidized in the presence of oxygen and the metal catalyst at 260xc2x0 C. and a contact time of 0.95 seconds, wherein the ratio of Mo:V:Sb:Nb is 1:0.3:0.23:0.08. A 97.2% yield of acrylic acid is achieved.
German Patent Publication No. 19815281 describes the preparation of molybdenum and/or vanadium containing mixed metal oxide catalysts for the partial oxidation of acrolein to acrylic acid. The mixed metal oxide catalysts have the formula ApBqCr, wherein A is Mo12VaX1bX2cX3dX4cX5fX6gOx, B is X7CuhHiOy, C is X8SbjHkOz, X1 is W, Nb, Ta, Cr, and/or Ce, X2 is Cu, Ni, Co, Fe, Mn, and/or Zn, X3 is Sb and/or Bi, X4 is Li, Na, K, Rb, Cs and/or H, X5 is Mg, Ca, Sr, and/or Ba, X6 is Si, Al, Ti, and/or Zr, X7 is Mo, W, V, Nb, and/or Ta, X8 is Cu, Ni, Zn, Co, Fe, Cd, Mn, Mg, Ca, Sr, and/or Ba, a is 1-8, b is 0.2-5, c is 0-23, d is 0-50, e is 0-2, f is 0-5, g is 0-50, h is 0.3-2.5, i is 0-2, j is 0.05-50, k is 0-50, the values of x, y, and z depend on the valence and relative abundance of the remaining elements in the composition, and p, q, and r are greater than or equal to zero, with the proviso that p/(q+r) is 1-20. The mixed oxide catalysts are prepared from finely ground precursor components which are mixed, dried, and calcined at 250-500xc2x0 C.
European Patent Publication Nos. 774297, 711745, 0685259, 427508, and 293859 relate to the use of mixed metal oxide catalysts containing molybdenum and vanadium in the gas phase oxidation of acrolein. WO 9627437 discloses the manufacture of a highly active catalyst having high mechanical strength suitable for the gas phase oxidation of acrolein. The catalyst contains molybdenum, vanadium, copper, and antimony as essential components. Similarly, U.S. Pat. No. 4,289,654 describes the use of a mixed metal oxide catalyst containing molybdenum, vanadium, tungsten, copper, and chromium on an alpha alumina support to oxidize acrolein to acrylic acid at 97% selectivity.
Japanese Patent Publication Nos. 05329371 and 05317713 relate to the production of a highly selective catalyst for acrolein oxidation. The catalyst is produced from a catalyst precursor containing molybdenum and vanadium, among other components. Japanese Patent Publication No. 04321642 describes the use of a molybdenum and vanadium based metal oxide catalyst to oxidize acrolein in the gas phase. The catalyst has the formula MoaVbSicNadAeXfYgOh, wherein A is Fe, Cr, Co, and/or Sr, X is Ge, Se, Sn, Te, and/or Sm, Y is Mg, Ti, Cu, Zn, Nb, Mn, Ni, Ag, Ta, Bi, Ce, and/or W, and a, b, c, d, e, f, and g are integers, such that the ratio of one to the other is a:b:c:d:e:f:g or 12:(1-6):(0.1-15):(0-2):(0.1-3): (0.1-3):(0-3), and h is an integer which satisfies the valance of the remaining elements of the composition. The patent describes the use of such a catalyst (having a formula Mo12V3Si4FeGe0.2Oh) with a gas mixture of 5% acrolein, 10% oxygen, 30% steam, and 55% nitrogen at 270xc2x0 C. to yield acrylic acid with 95% selectivity and at 99% conversion of acrolein.
Soviet Union Patent Publication Nos. 858916 and 899112 describe a catalyst containing vanadium oxide and molybdenum oxide, and copper oxide as a promoter on a silica support suitable for acrolein oxidation to acrylic acid. The catalyst exhibits increased activity and mechanical strength due, at least in part, to its method of manufacture. The catalyst is prepared by granulating the catalyst material after drying and adding promoter and a binder compound during granulation. Great Britain Patent Publication No. 1586173 discloses the use of metal oxide catalysts containing molybdenum, vanadium, chromium, copper, and a metal selected from Mb, Ta, and/or Ti, for the oxidation of acrolein to acrylic acid with improved conversion and selectivity. The ratio of molybdenum, vanadium, chromium, copper, and the metal is Mo:V:Cr:Cu:M or 15:(5-7):(0.7-0.8):(2.4-2.8):(1.5-2). A feedstock containing 5.6 mol % acrolein, 30 mol % nitrogen, and oxygen was passed through the catalyst and heated to 300xc2x0 C. in a 3.05 mxc3x972.54 cm stainless steel pipe reactor. At a contact time of 2.9 seconds, 99% of the acrolein was converted to acrylic acid at a selectivity for acrylic acid and carbon oxides of 93.4% and 4.4%, respectively.
The catalysts described above exhibit decreased stability over time (see European Patent Publication No. 0711745, Japanese Patent Publication No. 25914, and Cat. Rev. Sci. Eng., vol. 35, p. 213 (1993)). This is due to the generation of V2O5, which is responsible for the slow decay of the catalyst performance. Therefore, it would be desirable to develop a molybdenum and vanadium based catalyst for the production of carboxylic acids from aldehydes which would exhibit enhanced performance and stability.
The present invention provides a new highly selective catalyst system for the oxidation of aldehydes in the gas phase. The catalyst compositions of the present invention are particularly well suited for the catalytic preparation of alpha-beta unsaturated carboxylic acids from the corresponding aldehydes, e.g., the production of acrylic acid from acrolein. Using the improved catalyst, acrolein is oxidized in the presence of molecular oxygen at relatively high levels of conversion, selectivity, and productivity, at temperatures preferably from about 150xc2x0 C. to about 450xc2x0 C.
The catalyst compositions of the present invention include compositions of the formula:
MoaVbAlcXdYeOz
wherein:
X is at least one element selected from W and Mn;
Y is at least one element selected from Pd, Sb, Ca, P, Ga, Ge, Si, Mg, Nb, and K;
a is 1;
b is 0.01 to 0.9;
c is  greater than 0 to 0.2;
d is  greater than 0 to 0.5;
e is  greater than 0 to 0.5; and
z is the number of oxygen atoms required to satisfy the valency of Mo, V, Al, X, and Y in the composition. The catalysts are preferably produced using the methods disclosed herein.
One aspect of the invention relates to catalysts for the production of olefins from hydrocarbons via oxidative dehydrogenation. The catalyst composition comprises a compound of the formula:
MoaVbAlcXdYeOz
wherein:
X is at least one element selected from W and Mn;
Y is at least one element selected from Pd, Sb, Ca, P, Ga, Ge, Si, Mg, Nb, and K;
a is 1
b is 0.01 to 0.9;
c is  greater than 0 to 0.2;
d is  greater than 0 to 0.5;
e is  greater than 0 to 0.5; and
z is the number of oxygen atoms required to satisfy the valancy of the remaining elements in the composition.
The catalyst composition of the present invention can be used with or without a support. The choice of the components of the composition used, as well as the specific procedures used to prepare the catalyst composition, can have a significant effect on its performance. The components of the catalyst composition are in combination with oxygen as oxides.
Preferably, the catalyst compositions are prepared from a solution of soluble compounds (salts, complexes or other compounds) of each of the metals. The solution is preferably an aqueous system having a pH of 1 to 10, and more preferably a pH of 1 to 7, preferably at a temperature from about 30xc2x0 C. to about 100xc2x0 C. Water is removed, e.g., by filtration, to complete dryness. The catalyst is dried in an oven at a temperature, e.g., from 100xc2x0 C. to 130xc2x0 C. The dried catalyst is calcined by heating to a temperature from about 250xc2x0 C. to about 600xc2x0 C., preferably about 250xc2x0 C. to about 450xc2x0 C., in air or oxygen for about one hour to about 16 hours to produce the desired catalyst composition.
Suitable supports for the catalyst include alumina, silica, titania, zirconia, zeolites, silicon carbide, molybdenum carbide, molecular sieves and other microporous/nonporous materials, and mixtures thereof Support materials can be pretreated with acids, such as HCl, HNO3, H2SO4, per acids or heteropoly acids, and alkali base materials such as KOH or NaOH. When used on a support, the support usually comprises from about 5% to about 95% by weight, preferably from about 50 to 90% by weight of the catalyst composition, with from about 5% to about 50% or the remainder being the catalyst.
Preferably, molybdenum is introduced into the solution in the form of ammonium salts, such as ammonium paramolybdate, or organic acid salts of molybdenum, such as acetates, oxalates mandelates, and glycolates. Some other partially water soluble molybdenum compounds which may be used include molybdenum oxides, molybdic acid, and chlorides of molybdenum.
Preferably, vanadium, aluminum, gallium, silicon, germanium, antimony, phosphorus, niobium, potassium, magnesium, palladium, tungsten, and manganese are introduced into the catalyst slurry in the form of salts or acids, oxides, hydrates, acetates, chlorides, nitrate acetates, oxalates, and tartrates.
The catalysts of the present invention are preferably used in the oxidation of alpha-beta unsaturated aldehydes to the corresponding carboxylic acids. In a preferred embodiment, the feed mixture contains lower alkyl (preferably 2-6 carbon atoms, more preferably 2-6 carbon atoms) branched or straight chained unsaturated aldehydes. However, the method of the present invention may also be used for the oxidation of lower alkyl (preferably 2-6 carbon atoms, more preferably 3-6 carbon atoms) branched or straight chained alkanes or alkenes to the corresponding carboxylic acids, as well as for the ammoxidation of lower alkyl (preferably 2-6 carbon atoms, more preferably 3-6 carbon atoms) branched or straight chained alkanes or alkenes, such as methacrolein to methacrylic acid. In addition, feed comprising acrolein obtained by direct oxidation of propylene or propane containing by-products of carbon oxides, acrylic acid, acetic acid, acetone and acetaldehyde can also be used for such a reaction. These by-products are not harmful to the catalysts disclosed in the present invention.
The reaction mixture in the method is generally a gaseous mixture of 0.1 to 99 mol % unsaturated aldehydes, such as acrolein, 0.1 to 99 mol % molecular oxygen either as pure oxygen or in the form of air, zero to 50 mol % water in the form of steam, and 0 to 90 mol % nitrogen or another inert gas. The gaseous mixture is generally introduced into a reaction zone having a temperature of about 150xc2x0 C. to about 400xc2x0 C. or 450xc2x0 C., and preferably 170xc2x0 C. to 350xc2x0 C., more preferably 200xc2x0 C. to 300xc2x0 C. The reaction zone generally has a pressure of 15 to 400 psi or 1 to 50 bar, and preferably 1 to 30 bar, the contact time between the reaction mixture and the catalyst is about 0.01 second to 100 seconds, and preferably 0.1 second to 10 seconds, and the space hourly velocity is about 50 to about 50,000 hxe2x88x921, and preferably about 500 to about 20,000 hxe2x88x921 and more preferably from 100 to 10,000 hxe2x88x921. The process is generally carried out in a single stage fixed bed or fluidized bed reactor, or solid moving bed reactor wherein all the oxygen and reactants are supplied as a single feed, with non-reacted starting materials being recycled. However, multiple stage addition of oxygen to the reactor with intermediate hydrocarbon feed can be used. This may improve productivity and avoid potentially hazardous conditions such as hydrocarbon and oxidantexplosive mixture or mixture of hydrocarbon and oxidant in explosive envelope, generation of hot spots which ultimately affect the product distribution and catalyst life.
In one embodiment, the feed comprises a mixed gas containing about one to about 25% by volume alpha beta unsaturated aldehyde, about 0.25 to about 30% by volume oxygen, zero to about 50% by volume steam and about 10 to about 80% by volume inert gas and the process provides an alpha beta unsaturated aldehyde conversion of at least 98% with a selectivity to corresponding unsaturated carboxylic acid of at least 80%.
The following examples are intended to be illustrative of this invention. They are, of course, not to be taken to in any way limit the scope of this invention. Numerous changes and modifications can be made with respect to the invention without departing from the spirit or scope of the present invention.